Switched telecommunications networks, such as Local and Wide Area Data and Voice Networks, route transmission information (data and/or voice) over various transmission links connected to Data Terminal Equipments (DTEs). The transmission links transmit electrical signals between DTEs with transmission media such as two-wire open lines, twisted pair lines, coaxial cable, optical fibers, satellite systems, terrestrial microwave links, radial links, and so on.
Switched telecommunications networks may include public carrier networks, private carrier networks, or hybrid combinations of the same. Public carrier networks include networks intended primarily for voice communication, such as PSTN (Public Switch Telephone Network), and networks intended mainly for data communication such as PSDN (Packet-Switched Data Network) and ISDN (Integrated Services Digital Network). Private carrier networks have capabilities similar to those of public carrier networks, but are privately-owned and controlled.
Switched telecommunications networks in North America Europe and Japan utilize multiple channel transmission links, which transmit multiple channels of information in the form of a time-division-multiplexed (TDM) signal. In forming the TDM signal, sets (characters) of digital bits, with each set (character) corresponding to a respective channel of information, are interleaved in time by time-division-multiplexing (TDM); and the interleaved sets of samples are transmitted serially on a common bus which forms the multiple channel transmission link. Such multiple channel transmission links are known as primary rate carrier links. In North America and Japan, T1 primary rate carrier links are used in which 24 information channels are grouped together within each link; in Europe, PCM-30 primary rate carrier links are used in which 30 information channels are grouped together within each link. T1 and PCM-30 primary rate carrier links have aggregate bit rates of 1.544 Mbps and 2.048 Mbps, respectively.
T1 and the PCM-30 links transmit information in the form of character (byte)-interleaved serial digital bit streams, and are used to form transmission links in a switched telecommunications network. A typical switched telecommunications network includes, among several entities, a central office, a remote office, and one or more repeaters. The repeaters are disposed between the central office and the remote office, and regenerate signals passing therethrough, to thereby extend the transmission distance between the central and remote offices.
Remote testing of the transmission link, and of the repeaters and other devices placed along the link, is typically facilitated by placing a loopback mechanism within the repeaters and other devices placed along the link. The operation of a loop-back mechanism will be described more further below.
FIGS. 1-3 illustrate how TDM links such as T1 and PCM-30 links are produced, and provide a context (by way of example) in which to describe prior loopback systems and the loopback system of the present invention.
FIG. 1 illustrates a system for converting several input channels (24 channels with T1, and 30 channels with PCM-30) of channel information into a TDM signal that comprises a byte-interleaved serial digital bit stream. A plurality of coder circuits 100 are provided. Each coder circuit 100 corresponds to a particular channel of transmission information. The output of each coder circuit 100 is connected to a corresponding input of a time division multiplexer 102. Time division multiplexer 102 manipulates transmission information which is input from coder circuits 100, and provides each of a plurality of buffers 104 with a byte of data corresponding to a respective coder circuit 100. Each byte is stored in a buffer 104, and is then assigned a specific time slot within the byte-interleaved serial digital bit stream.
FIG. 2 illustrates a D4 framing structure of a T1 link. As illustrated in FIG. 2, the byte-interleaved serial digital bit stream of a T1/D4 transmission link is arranged in accordance with a framing schedule in which 24 channels 106 (each channel corresponding to a byte) are consecutively arranged in the form of a frame 108. A channel 106 is illustrated in the top portion of FIG. 2, a frame 108 is illustrated in the middle portion of FIG. 2, and a superframe 110, which includes 12 frames, is illustrated in the bottom portion of FIG. 2. The consecutively numbered bits, channels, frames and superframes of the bit stream are transmitted in time from left to right, thus resulting in a direction of transmission as indicated by the arrow at the bottom of FIG. 2.
The first through seventh bits of each channel 106 comprise transmission information, i.e., link payload information. The eighth bit (the least significant bit) of each channel 106 (called a signalling bit) is used either for supervision, or signalling (e.g. to establish a connection, or to terminate a call). The eighth bit of selected channels in every sixth and twelfth frame of superframe 110 contains signalling information. These signalling bits are inserted by "robbing" the eighth bit of each data word of each channel 106, and by replacing the "robbed" eighth bit with a signalling bit. This mechanism is referred to as "robbed bit signalling."
The 193rd bit of each frame is referred to as the multiframe alignment bit or "F" bit. The "F" bit may comprise a bit for terminal framing, designated as Ft, or a bit for multiframe synchronization (used to identify frames 6 and 12), designated as Fs.
FIG. 3 illustrates a CEPT PCM-30 transmission framing format. The first channel 106 (which corresponds to time slot zero) and the seventeenth channel 106 (time slot sixteen) facilitate the transmission of signalling information, such as on hook and off hook, call progress, dialing digits, and so on. For "even" frames, within the sixteen frame multiframe structure (superframe 110), time slot zero is utilized to indicate a frame alignment signal (FAS). During the alternate "odd" frames of the superframe 110, time slot zero contains international bits I, national bits N (reserved for national use by respective countries), and an alarm indication signal A. Time slots 1-15 and 17-31 are assigned to 30 telephone channels numbered 1 to 30, and all eight bits of each time slot represent transmission information.
As noted above, remote testing of transmission links, and of repeaters and other devices placed along the link, is typically facilitated by providing a loopback mechanism within each of the repeaters and other devices placed along the link.
Loopback systems are commonly used for maintenance testing of T1 and PCM-30 transmission links, as well as other types of transmission links. A T1 loopback system is described in U.S. Pat. No. 5,010,544 (CHANG et al.) which utilizes an in-band technique for transmitting loopback control information. As described therein, a test unit (usually located in a central office) sends a particular code (known as a loop-up code) to a loopback mechanism, which is provided in a device placed along the link (e.g., a repeater). In response to the loop-up code, the loopback mechanism loops the communications link back on itself, so that the same test unit can send a signal out on the T1 line and measure that signal as it comes back. The test unit sends the signal along one of the two pairs of the T1 line, and the signal is then looped back along the other pair of the T1 line. The loopback mechanism is restored to its normal condition, i.e., the loopback is "dropped," by sending a loop-down code. The loop-up and the loop-down commands are sent over the tested T1 line by a test unit that has intrusive access to the line. The loopback system disclosed in U.S. Pat. No. 5,010,544 (CHANG et al.) is an in-band system, i.e., it utilizes an "in-band" T1 loopback instruction set.
In such in-band loopback systems, in-band channels, normally occupied by transmission information, are infringed upon. Accordingly, all of the transmission information is not transmitted over the link when the loopback testing is performed, and the system is not tested under real conditions. In addition, since the loopback control codes are sent in-band, the system may mistakenly be placed in a loopback state if a predetermined sequence of bits occurs randomly within the transmission (payload) information.
In some systems, transmission of the link's payload is stopped or postponed for purposes of performing a loopback. The resulting down time is obviously undesirable. In addition, as is the case with in-band loopback systems, the test is not performed under normal data transmission conditions.
Transmission links that are provided with loopback systems include a mechanism for transmitting loopback command information from a test unit to a loopback mechanism provided within a device placed along the link. There are disadvantages associated with conventional and/or prior loopback systems in that the loopback control information is transmitted in-band (i.e., within channels intended for transmission of payload information), or the transmission of payload information must be shut down during loopback, to allow the loopback control information to be transmitted.
Another feature of conventional and/or prior loopback systems is that the test unit identifies the device at which a loopback is to be performed by referring to the device's address or to a number that has been assigned to the device. Systems of this type are limited in that the system will have to be reconfigured (by, e.g., updating a data-base or inventory list) upon introduction of a new loopback-capable device along the link, so that the test unit knows the new device's number or address.